AD9761ARS datasheet(17/23 Pages) AD

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AD9761

–17–

REV. A

Maintaining low noise on power supplies and ground is critical

to obtaining optimum results from the AD9761. If properly

implemented, ground planes can perform a host of functions on

high speed circuit boards: bypassing, shielding, current trans-

port, etc. In mixed signal design, the analog and digital portions

of the board should be distinct from each other, with the analog

ground plane confined to the areas covering the analog signal

traces and the digital ground plane confined to areas covering

the digital interconnects.

All analog ground pins of the DAC, reference and other analog

components should be tied directly to the analog ground plane.

The two ground planes should be connected by a path 1/8 to 1/4

inch wide underneath, or within 1/2 inch of the DAC to main-

tain optimum performance. Care should be taken to ensure that

the ground plane is uninterrupted over crucial signal paths. On

the digital side, this includes the digital input lines running to

the DAC as well as any clock signals. On the analog side, this

includes the DAC output signal, reference signal and the supply

feeders.

The use of wide runs or planes in the routing of power lines is

also recommended. This serves the dual role of providing a low

series impedance power supply to the part, as well as providing

some “free” capacitive decoupling to the appropriate ground

plane. It is essential that care be taken in the layout of signal and

power ground interconnects to avoid inducing extraneous volt-

age drops in the signal ground paths. Its is recommended that

all connections be short, direct and as physically close to the

package as possible, in order to minimize the sharing of conduc-

tion paths between different currents. When runs exceed an inch

in length, strip line techniques with proper termination resistor

should be considered. The necessity and value of this resistor

will be dependent upon the logic family used.

For a more detailed discussion of the implementation and con-

struction of high speed, mixed signal printed circuit boards,

refer to Analog Devices’ application notes AN-280 and AN-333.

APPLICATIONS

Using the AD9761 for QAM Modulation

QAM is one of the most widely used digital modulation schemes

in digital communication systems. This modulation technique

can be found in both FDM as well as spread spectrum (i.e.,

CDMA) based systems. A QAM signal is a carrier frequency

that is modulated both in amplitude (i.e., AM modulation) and

in phase (i.e., PM modulation). It can be generated by indepen-

dently modulating two carriers of identical frequency but with a

° phase difference. This results in an in-phase (I) carrier com-

ponent and a quadrature (Q) carrier component at a 90

° phase

shift with respect to the I component. The I and Q components

are then summed to provide a QAM signal at the specified car-

rier frequency.

A common and traditional implementation of a QAM modula-

tor is shown in Figure 41. The modulation is performed in the

analog domain in which two DACs are used to generate the

baseband I and Q components, respectively. Each component is

then typically applied to a Nyquist filter before being applied to

a quadrature mixer. The matching Nyquist filters shapes and

limits each component’s spectral envelope while minimizing

intersymbol interference. The DAC is typically updated at the

QAM symbol rate or possibly a multiple of it if an interpolating

filter precedes the DAC. The use of an interpolating filter typi-

cally eases the implementation and complexity of the analog

filter which can be a significant contributor to mismatches in

gain and phase between the two baseband channels. A quadra-

ture mixer modulates the I and Q components with in-phase

and quadrature phase carrier frequency and then sums the two

outputs to provide the QAM signal.

CARRIER

FREQ

NYQUIST

FILTERS

QUADRATURE

MODULATOR

MIXER

IOUT

QOUT

AD9761

DSP

ASIC

Figure 41. Typical Analog QAM Architecture

EVALUATION BOARD

The AD9761-EB is an evaluation board for the AD9761 dual

10-bit, 40 MSPS DAC. Careful attention to layout and circuit

design along with prototyping area, allows the user to easily and

effectively evaluate the AD9761. This board allows the user the

flexibility to operate each of the AD9761 DACs in a single-

ended or differential output configuration. Each of the DACs’

single-ended outputs are terminated in a 50

Ω resistor. Evaluation

using a transformer coupled output can be accomplished simply

by installing a Minicircuit transformer (i.e., Model T2-1T) into

the available socket.

The digital inputs are designed to be driven directly from vari-

ous word generators with the onboard option to add a resistor

network for proper load termination. Separate 50

Ω terminated

SMA connectors are also provided for the CLOCK, WRITE

and SELECT inputs. Provisions are also made to operate the

AD9761 with either the internal or an external reference as well

as to exercise the power-down feature.

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